The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A nerve of a patient may be stimulated by applying current to the nerve via a mono-polar stimulation probe. The mono-polar stimulation probe may include a stimulating electrode tip. A surgeon may touch a location on a patient with the electrode tip to provide a voltage and/or current to a location on the patient and stimulate nerve activity and as a result a muscle response (or muscle activity). A return (or anodal) needle may be attached: via a wire, to the mono-polar stimulation probe; and to the patient away from (i) sensors, and (ii) an area being stimulated. The sensors can include electrodes that are attached to the patient and used to monitor the muscle activity. Although the mono-polar stimulation probe is capable of providing deep focused penetration of applied current to a nerve, the mono-polar stimulation probe restricts movement of a hand of surgeon due to the attachment of the mono-polar stimulation probe to the return needle.
To eliminate use of the wire and return needle, a concentric probe, a side-by-side bipolar stimulation probe or a tri-polar stimulation probe may be used. The concentric probe includes an anodal (or central) electrode that extends within cathodal electrode. The anodal electrode is isolated from the cathodal electrode via an insulative shield around the anodal electrode. Although the concentric probe eliminates need of the wire and return needle associated with the mono-polar stimulation probe, current density and current tissue penetration is low.
The side-by-side bipolar stimulation probe and the tripolar stimulation probe are similar. The side-by-side bipolar stimulation probe includes two tips (an anodal electrode and a cathodal electrode). The tripolar stimulation probe has three tips (two cathodal electrodes and a single anodal electrode). The anodal electrode is positioned between the two cathodal electrodes. The tripolar stimulation probe is approximately 30% larger in size than the side-by-side bipolar stimulation probe due to the extra (or third) electrode.
The side-by-side bipolar stimulation probe has a single anodal electrode and a single cathodal electrode. Electrical current flowing through the two electrodes may be directly or indirectly applied to a nerve to stimulate the nerve. A negative electrical current may be applied to the nerve via the cathodal electrode (referred to as a cathode or negative electrode). The nerve resists excitation at the anodal electrode (referred to as an anode or positive electrode). This is a result of negative current from the cathode reducing voltage outside a neuronal cell membrane of the nerve, causing depolarization and an action potential. The anode injects positive current outside the neuronal cell membrane, which leads to hyperpolarization. Preferential cathodal stimulation refers to a reduced amount of current (one third to one quarter) needed to elicit a motor response of a muscle when the cathode is used as the stimulating electrode. The amount of current applied when the cathode is used is less than an amount of current needed to elicit a motor response of a muscle when the anode is used as the stimulating electrode. In order to stimulate a nerve using the cathode: the cathode may be attached to a stimulating needle or catheter; and the anode may be used as a current returning electrode and be attached to or in contact with the skin of the patient via a return wire.
When a surgeon uses a side-by-side bipolar stimulation probe, orientation of the electrodes of the side-by-side bipolar stimulation probe relative to a nerve influences an evoked response associated with the stimulation of the nerve. A nerve action potential evoked by the stimulation differs depending on the orientation of the electrodes relative to the nerve. The cathode of the bipolar stimulation probe must be placed distally along a nerve to evoke a proper response. In placing the cathode distally along the nerve, the cathode, relative to the anode, is directed away from an axonal head (or cell body) of a nerve and toward axon terminals of the nerve and/or a target muscle. If not oriented properly, no response or an improper response (e.g., an erratic signal or signal with low signal strength) may be generated.
Although electrodes of a side-by-side bipolar stimulation probe must be oriented properly relative to a nerve to obtain a proper response and to minimize an amount of current applied to receive a proper response, the electrodes may be improperly oriented for various reasons. For example, a surgeon may not be aware that the electrodes of the side-by-side bipolar stimulation probe needs to be oriented properly relative to a nerve. As another example, a surgeon may not be aware of an orientation of a nerve and as a result may not be aware of where an axonal head or a distal end of a nerve exists. For this reason, the surgeon may not be able to determine a proper orientation of electrodes of a side-by-side bipolar stimulation probe. As yet another example, a surgeon may not be aware of an orientation of electrodes of a bipolar stimulator on a nerve because of anatomical variation of nerves of a patient. Also, a surgeon may inadvertently change an orientation of electrodes of a side-by-side bipolar stimulation probe by simply rotating the side-by-side bipolar stimulation probe in a hand of the surgeon. These human factors may result in the side-by-side bipolar stimulation probe failing to evoke a proper response from a nerve. As a result, a surgeon may inadvertently resect nerve tissue that is thought not to be nerve tissue due to a negligible muscle response and/or lack of a detected muscle response.